The velocity–time graph of a cart moving in a straight line consists of two segments: from t=0 to t=3s, the velocity is +4m/s; from t=3s to t=6s, the velocity is −2m/s. The cart’s initial position is x=0. What is the cart’s displacement at t=6s?
(a) +12m
(b) +6m
(c) −6m
(d) +18m